1. Field of the Invention
The present invention relates to novel apparatus and processes for the injection of an oxidant into a rotary kiln. More particularly, the present invention relates to apparatus and processes which significantly improve combustion in a rotary kiln used for the calcination of minerals such as cement, lime, dolomite, magnesia, titanium dioxide, and other calcined materials.
2. Brief Description of the Related Art
The introduction of oxygen into a combustion space, e.g., a furnace, is used in a variety of industries for enhancement of the combustion process. The general use of oxygen in cement rotary kilns has been shown to lead to a significant production increase of the kiln, starting with the work of Gaydas, R. A., "Oxygen enrichment of combustion air in rotary kilns," Journal of the PCA R & D Laboratories, 49-66 (September 1965) (incorporated by reference in its entirety herein). Gaydas presents test results from a period between 1960 and 1962. It is noted that Geissler suggested in 1903 that oxygen be used for clinker production. Experimental work was done in Germany in the 1940's, but results are not available.
To date, the use of oxygen in rotary kilns has been applied in three main ways, well documented in the literature: introducing oxygen into the primary air, i.e., into the main burner; the utilization of an oxy-fuel burner in addition to a standard air-fuel burner; and oxygen lancing into the rotary kiln, particularly in a region between the load and the flame, for improved flame characteristics. One of the more documented uses of oxygen in rotary kilns is described in Wrampe, P. and Rolseth, H. C., "The effect of oxygen upon the rotary kiln's production and fuel efficiency: theory and practice", IEEE Trans. Ind. App., 568-573 (November 1976) (incorporated by reference in its entirety herein), which indicates that production increases above 50% produce excessive temperatures in the kiln, but below this level, kiln operation takes place without major problems.
Each method of introducing oxygen into the cement plant has advantages, as well as disadvantages. The introduction of oxygen into the primary air limits the total amount of oxygen capable of being introduced into the kiln, as modern cement kilns utilize 5-10% of the total air used as primary air. Therefore, in order to introduce a meaningful amount of oxygen into the kiln, it is necessary to significantly increase the concentration of oxygen in the air-fuel stream. Increasing the oxygen concentration may lead to potential safety problems, since the fuel is in contact with the O.sub.2 enriched air prior to its arrival into the kiln's combustion space, and therefore can burn too early, or explode.
The use of a separate oxy-burner represents a more involved solution to increase the thermal transfer to the load, which in general requires significant quantities of quality fuel, such as natural gas or oil, as well as important modifications in the kiln back wall. This method has been previously proposed, such as U.S. Pat. No. 3,397,256 (which is incorporated by reference in its entirety herein). The use of oxygen lances, although a more elegant solution, can locally increase the temperature of the combustion space, which can result in non-uniform heat transfer to the entire flow of clinkers moving through the kiln. Oxygen lances can produce hot spots in the refractory, which may damage the refractory. Further, the introduction of cold oxygen may limit the beneficial effect of oxygen on combustion by locally cooling the flame. The employment of oxygen lances has been proposed in U.S. Pat. No. 5,572,938, U.S. Pat. No. 5,007,823, U.S. Pat. No. 5,580,237, and U.S. Pat. No. 4,741,694, all of which are incorporated by reference in their entireties herein.
U.S. Pat. No. 4,354,829 ('829 patent) describes mixing air and oxygen in a separate pipe, and introducing it through the rotary kiln moving walls. This device suffers from a number of significant problems, including the difficulty of creating a leak-free plenum which rotates with the kiln and the difficulty of installing tubes into the kiln. Additionally, the procedure disclosed in the '829 patent suffers from certain inherent drawbacks. Injecting an oxygen-enriched air mixture of 23-25% oxygen injects a significant amount of nitrogen into the kiln. Nitrogen does not facilitate combustion, and thus represents an unproductive use of flue gas volume. Excessive nitrogen content may result in additional dust generation and may significantly increase the amount of nitrous oxide (NO.sub.x) emissions. Also, the injection of cold ambient air may reduce the thermal efficiency of the kiln and may cause additional stresses in the rotary kiln which can damage its expensive structure from thermal shock.
It is an object of the present invention to provide a safe, yet efficient system and method of introducing oxygen into rotary kilns used, for example, in cement producing equipment, in a manner which will enhance flame characteristics and improve production without adversely affecting overall plant operation.